There are a number of inorganic battery technologies which offer advantages for the storage of energy in large scale systems. There are many types of molten salt batteries, of which sodium-sulfur is currently most important of these technologies. A sodium-sulfur battery is a type of battery constructed from sodium (Na) and sulfur (S). This type of battery has a high energy density, high efficiency of charge/discharge (99.0-99.6% Coulombic Efficiency) and long cycle life, and is fabricated from inexpensive materials. However, because of the operating temperatures of 110 to 350° C., based on the melting point of sodium, and the highly corrosive nature of the sodium polysulfides, such cells can be difficult to make practical in many applications. Most batteries operate more efficiently when operated at elevated temperatures. Some batteries use sodium requiring a molten state for battery operation. Naturally, sodium-sulfur batteries are favored for large utility scale energy storage systems where long-life of sodium-sulfur battery justify the cost of heating. Sodium-sulfur batteries require an operating temperature between 110° C. to 350° C. For optimum performance, current sodium-sulfur cell technology cells are normally operated between 290° C. and 340° C.
Another related battery technology is called the Zebra cell. The Zebra battery operates at 250° C. (482° F.) and utilizes molten sodium chloroaluminate (NaAlCl4), which has a melting point of 157° C. (315° F.), as the electrolyte. Here again, the battery cells' lower operating temperature is limited by the melting temperature of the sodium chloroaluminate. Batteries produced from cells based on these technologies are difficult to make practical in many applications and normally result in lost energy required by continuous heating.
The cell of a sodium-sulfur battery is usually made in a tall cylindrical configuration. The entire cell is enclosed by a steel casing that is protected, usually by chromium and molybdenum, from corrosion on the inside. This outside container serves as the positive electrode, while the liquid sodium serves as the negative electrode. The container is sealed at the top with an airtight alumina lid. An essential part of the cell is the presence of a BASE (beta-alumina sodium ion exchange) membrane, which selectively conducts Na+. The cell becomes more economical with increasing size. In commercial applications the cells are arranged in blocks for better conservation of heat and are encased in a vacuum-insulated box.
During the discharge phase, molten elemental sodium at the core serves as the anode, meaning that the Na donates electrons to the external circuit. The sodium is separated by a BASE cylinder from the container of sulfur, which is fabricated from an inert metal serving as the cathode. The sulfur is absorbed in a carbon sponge. BASE is a good conductor of sodium ions, but a poor conductor of electrons, so avoids self-discharge. When sodium gives off an electron, the Na+ ion migrates to the sulfur container. The electron drives an electric current through the molten sodium to the contact, through the electrical load and back to the sulfur container. Here, another electron reacts with sulfur to form Sn2−, sodium polysulfide. The discharge process can be represented as follows:2Na+4S→Na2S4Ecell˜2V
As the cell discharges, the sodium level drops. During the charging phase the reverse process takes place. Once running, in a well-designed battery system, the heat produced by charging and discharging cycles can be sufficient to maintain operating temperatures and usually no external source is required.
Previous sodium-sulfur batteries rely on self-heating of the battery, which is generally available only during charge or discharge cycles. Further, the charge or discharge rate is often determined by the requirements of the external power system, and often it is not practical to change charge or discharge rates solely for battery temperature control. A separate source of heat is therefore generally employed, often either an electric heater or a solar heater. Electric heating of the battery consumes energy that would otherwise be available to distribute. Solar heating relies on exposure to sunlight which can be inconsistent, and can limit the size of the battery array, and can cause undesirable thermal cycling of the batteries.
The challenges posed by the required high operating temperatures are evident from U.S. Pat. No. 6,958,197. In that patent, a special control system was used to minimize the time lag between charge and discharge cycles so that self-heating of the battery was maintained, and power consumption of separate battery heaters could be reduced.
The challenge of preventing battery cell damage by over-temperature condition is an issue for all batteries including sodium-sulfur. In standard sodium-sulfur batteries the operating temperatures can go as high as 390° C., however, because of the corrosive nature of the materials, the upper temperature range is controlled to a peak temperature of 350° C. to reduce excessive corrosive damage. In energy storage systems using standard sodium-sulfur batteries, an over-temperature condition requires the batteries to be disconnected and allowed to cool for several hours before the energy storage system can be reactivated. This is also true for Zebra batteries and all types of molten battery technologies.
Sodium-sulfur batteries have been proposed as especially suitable for energy storage in electric power applications, where variation in demand for energy can require generation and transmission capability to meet peak demands, while the average demand is much less. For example as in U.S. Pat. No. 6,522,103, in a sodium-sulfur battery system comprising a battery module having a sodium-sulfur battery contained in a thermal insulation container, an amount of peak-shift of an electric power line, which can be performed by the battery module, is calculated using a daily load characteristic of the electric power line and a discharge characteristic of the battery module, and an allowable amount of heat generation in battery and an allowable amount of discharge, and discharge of the battery module is controlled using the calculated result. The batteries were contained in thermal insulation containers to facilitate maintenance of the required high operating temperatures.
The need for effective storage can be even greater in connection with energy sources such as wind and solar, where the power output from the generator can also vary. A significant challenge to the use of sodium-sulfur batteries for such energy storage applications is the efficient provision and management of the heat required to maintain the battery's required operating temperature.
Geothermal heat sources, which produce geothermal fluids, are also used as energy sources to generate electricity. Here, geothermal fluids are used to drive a turbine to produce electricity. Geothermal power production is created as a function of heat removal from the geothermal fluids. Production wells feed high temperature geothermal fluids to the power plant and injection wells return the now cooler geothermal fluid from the plant back to the earth's reservoir. Power output can be more consistent but may still not effectively accommodate variations in demand. Therefore, there is still a need for effective energy storage.
Geothermal fluids include water or brines, oil, natural gas, CO2 or any combination of co-produced fluids as products produced or stored from wells drilled into earth reservoirs. For the purposes of this application, geothermal fluids also include any secondary liquids or gases heated by geothermal fluids. Geothermal fluids also include fluids being returned to reservoirs found in the earth.
Sodium-sulfur cells are combined in to large batteries to create the voltage (V) and current (A) needed by the electrical system. Utilized-sized batteries are very large with hundreds of cells. The normal sodium-sulfur cell provides approximately 2.1V and approximately 10 A of current. Large utilized-sized battery use stacks of cells (connections in series) to create 400 to 500V output. To create enough current, normally 100 A or more, parallel stacks of cells are used. A battery using an array of cells is electrically illustrated in FIG. 2A. Utilized-sized batteries place the cells in close physical arrangement to reduce the wiring lengths and inside a protective insulated enclosure to maintain the elevated temperatures require by the sodium-sulfur cells.
All batteries have internal impedance to the charging and discharging currents. This impedance results in a loss of energy in the form of heat. All batteries self-heat when being charged and discharged. For example, charging a 1 kilowatt (kW) battery would result in a loss of approximately 5% or 50 W of self-heating. The same would be true for the discharge of a 1 kW battery. The act of storing and then utilizing 1 kW of energy from a battery results an energy loss of 100 W. For conventional sodium-sulfur battery storage systems, this waste heat is used to help maintain the batteries elevated temperatures. Significant research in using the waste heat in sodium-sulfur batteries has been exhausted toward this effort because of a serious complication with insulating the battery to capture waste heat.
The complication of using the waste heat generated during charging and discharging of the sodium-sulfur batteries comes from the serious problem of overheating. Overheating of the battery will result in loss of operating life time meaning the system will require early replacement or complete destruction of the sodium-cells. As such, sodium-sulfur battery energy storage systems must shut down and be allowed to cool if full capability of the energy storage system is being utilized. Cooling of these large systems takes hours. Battery-based energy storage systems are rated for total energy storage capability along with the maximum energy rate versus time duration before thermal shut down.
All battery-based energy storage systems, including sodium-sulfur batteries, require power electronic circuits to control charging, discharging and to interface the battery energy to the customer. In existing sodium-sulfur energy storage systems, the electronic control and interface circuits are housed in a separate enclosure, away from the hot batteries.
All electronic devices create waste heat as a function of internal resistance and current. This is especially true of power electronic devices which handle high voltages and currents. The operating life of normal power electronics is a function of leakage current and metallization of the silicon electronic devices. Leakage currents cause excessive heat in high voltage operations. Metallization is the metal conductor junction to the silicon chip. High current densities and elevated temperatures at the metal junction cause metal migration, or metal atoms moving into the silicon, weakening the conductivity of the circuit and increasing resistance which increases the generation of waste heat in the electronics. If the temperatures are not controlled, the device will surfer catastrophic damage.
The total waste heat generated in using battery-based energy storage is a function of waste heat generated in the batteries and the power electronics. In general, the charging and discharging efficiency of a battery-based energy storage system is referenced as the round-trip efficiency, including the heat loss of the cell during charging and discharging, current leakage, power electronic conversion losses to heat, and the heating of the battery in the cases where charge/discharge is not sufficiently frequent to maintain the battery temperature in the optimum range. The round-trip efficiency is normally 67 to 74%. For very large scale energy storage systems needed by the utility industry this is a significant cost to pay. For a 50 megawatt (MW) system, a round-trip energy return is only 33.5 MW to 37 MW. In other words, approximately 15 MW of waste heat can be generated.
High-temperature electronics components are electronic devices produced with SOI (Silicon-On-Insulator), SOS (Silicon-On-Sapphire), SiC (Silicon-Carbide), GaN (Gallium-Nitride) or other wide bandgap materials. SOI and SOS reduce the leakage current produced when silicon electronic devices are exposed to elevated temperatures by building the circuit transistors on a nonconductive base material as silicon-oxide, intrinsic silicon or sapphire among others. Leakage current is reduced by a factor of 100. Metallization of these devices uses large conductive pads built with high density metals to greatly reduce current density and loss of electrical connection through metal migration.
High-temperature electronics use advanced circuit interconnections based on ceramic substrates or ceramic circuit boards not found in conventional power electronic circuits. Ceramic circuit boards include SiC ceramic with a very high thermal conductivity. This invention is enabled, in part, by developments in high-temperature circuit board designs developed for geothermal well monitoring systems by the inventor and others.
High-temperature electronics developed for geothermal well monitoring encompass complete solutions for all electronic components and hardware as geothermal wells produce fluids at temperatures of 100 to 350° C. without any place for self generated waste heat from electronic devices to go other than into the hot fluid. As such, the electronics must operate at elevated temperatures at all times and dissipate waste heat in to the hot ambient environment of the geothermal well.
Although present devices are functional, they are not sufficiently accurate or otherwise satisfactory. Accordingly, a system and method are needed to address the shortfalls of present technology and to provide other new and innovative features.